1. Field of the Invention
The present invention relates to an active matrix substrate including a source line driving circuit. More particularly, the present invention relates to an active matrix substrate including a source line driving circuit with an amplifier at the final output stage for outputting data to a source bus line, for example.
2. Description of the Background Art
Referring to FIG. 14, a liquid crystal display device with a built-in driving circuit will be described as an example of a conventional electrooptical device with a built-in driving circuit. The liquid crystal display device includes a plurality of pixel sections 3 arranged in a matrix pattern on a substrate, each pixel section 3 including a picture element transistor 1 and a storage capacitor 2 connected to the picture element transistor 1 for storing a charge. Gate bus lines 6 and source bus lines 9 are arranged so that they extend perpendicular to each other. The gate of the picture element transistor 1 is connected to the gate bus line 6, and the source of the picture element transistor 1 is connected to the source bus line 9. On the other hand, one terminal of the storage capacitor 2 that is not connected to the picture element transistor 1 is connected to a common electrode line 7 extending in a direction parallel to the gate bus line 6, i.e., perpendicular to the source bus line 9, and the common electrode line 7 is connected to one terminal 16.
The liquid crystal display device is driven as follows for displaying an image. A gate line driving circuit 5 outputs an ON signal successively to the rows of gate bus lines 6 to turn ON the picture element transistor 1 of all the rows of gate bus lines 6 to which the ON signal is output. Moreover, while the ON signal is being output to a gate bus line 6, a source line driving circuit 8 outputs an ON signal successively to analog switches 10 provided along the source bus lines 9, respectively. Thus, the source bus line 9 that is connected to the analog switch 10, being turned ON, is connected to a corresponding video signal line 12, thereby turning ON the picture element transistor 1 via the source bus line 9. The video signal from a terminal 13 is written, via the picture element transistor 1, to the storage capacitor 2 and to a picture element capacitor in a liquid crystal layer (not shown) between an active matrix substrate and a counter substrate.
Moreover, the video signal, which is thus written to the storage capacitor 2 and to the picture element capacitor is held by turning OFF the picture element transistor 1 while the gate line driving circuit 5 is outputting the ON signal to the other rows of gate bus lines 6. Then, after the gate line driving circuit 5 outputs the ON signal to all of the rows of gate bus lines 6, the above operation is repeated by outputting the ON signal successively starting from the first row.
The active matrix substrate can easily be inspected for defects through an optical inspection process after the active matrix substrate is attached to the opposing counter substrate via the liquid crystal layer therebetween so that the assembly can operate as a liquid crystal display device (see, for example, Japanese Laid-Open Patent Publication No. 63-123093). However, such an inspection method requires each liquid crystal panel to actually display a picture thereon, and thus takes a long time, whereby a high productivity cannot be expected. Moreover, with this inspection method, if an active matrix substrate is determined to be defective, the entire liquid crystal panel needs to be disposed of, whereby the assembly step of attaching the active matrix substrate to the counter substrate and the liquid crystal injection step may possibly be wasted completely. Therefore, it is desired to inspect an active matrix substrate after the picture element transistors 1, etc., are formed thereon, so that it can be forwarded to the assembly step of attaching the active matrix substrate to the counter substrate after correcting any defects, if possible.
One way to inspect an active matrix substrate before the assembly step is to form inspection circuits 111 to 114, as illustrated in FIG. 15, on the substrate. The inspection circuits 111 and 112 are circuits for leading the outputs at the last stage of shift registers in a gate line driving circuit 105 and a source line driving circuit 106 to inspection pads 111a and 112a, respectively. Thus, the gate line driving circuit 105 and the source line driving circuit 106 can be inspected by operating the circuits 105 and 106 while monitoring their outputs at the inspection pads 111a and 112a. 
Moreover, the inspection circuit 113 is a circuit by which gate bus lines 101 are all connected to an inspection pad 113b via respective switches 113a. Furthermore, the inspection circuit 114 is a circuit by which source bus lines 102 are all connected to an inspection pad 114b via respective switches 114a. 
The switches 113a and the switches 114a are turned ON/OFF by a signal from an inspection pad 113c and by a signal from an inspection pad 114c, respectively. Therefore, the gate bus line 101, for example, can be inspected as follows. The gate line driving circuit 105 is operated, with an ON signal being applied to the inspection pad 113c to turn ON the switch 113a, whereby a defect such as a line break can be detected based on the output of the inspection pad 113b. 
Similarly, the source bus line 102 can be inspected as follows. The source line driving circuit 106 is operated with an appropriate signal being applied to a video signal line 108 and an ON signal being applied to the inspection pad 114c to turn ON the switch 114a, whereby a defect such as a line break can be detected based on the output of the inspection pad 114b. 
With this inspection method, however, it is possible to inspect only the operation of the gate line driving circuit 105 and the source line driving circuit 106, and the condition of the gate bus line 101 and the source bus line 102. Since a large number of picture element transistors 104 are formed on an active matrix substrate, the production yield can be better improved by inspecting the condition of the picture element transistors 104.
A method for inspecting not only the condition of the driving circuit and the bus lines but also the condition of the picture element transistors is disclosed in, for example, Japanese Laid-Open Patent Publication No. 5-5866. With this method, data that has been written in each picture element storage capacitor is read out and checked. In this way, it is possible to inspect not only the condition of the driving circuit and the bus lines but also the condition of the picture element transistors, and any defective location can reliably be detected. The inspection method disclosed in this publication will now be described with reference to FIG. 16 and FIG. 17.
FIG. 16 illustrates an active matrix substrate with a built-in driving circuit, and FIG. 17 illustrates a system for inspecting a picture element defect in the active matrix substrate of FIG. 16. The gate line driving circuit 305 of the active matrix substrate 300 is operated while receiving external control signals via terminals 315. Similarly, a source line driving circuit 306 is operated while receiving external control signals via terminal 314.
First, the write method will be described. For example, the gate line driving circuit 305 selects a gate line 301a to turn ON a picture element transistor 304. Furthermore, a video signal from an external signal source 418 is output to a video line 308a via a selector switch 412 and a terminal 313a to turn ON an analog switch 307 of a source line 302a that is selected by a source line driving circuit 306, thereby writing the video signal to a storage capacitor 303 of the intended picture element. One electrode of each storage capacitor 303 that is away from the picture element transistor 304 is connected with others via a common electrode line 310, and in turn to an external common power supply via a common electrode terminal 312. Therefore, a charge corresponding to the difference between the voltage of the common power supply and that of the video signal is written to the storage capacitor 303.
Next, the read-out method will be described. The selector switch 412 of the external circuit is turned from the side of the signal source 418 to the side of an analog amplifier 413. The charge stored in the storage capacitor 303 of the picture element is read out to the outside of the panel as the picture element transistor 304 along the selected gate line is turned ON, and the analog switch 307 along the selected source line is turned ON. The charge read out to the outside of the panel is subjected to current-voltage conversion and to voltage amplification at the analog amplifier 413. Then, the analog signal is converted into a digital signal at an AD converter 414, and the obtained digital signal is subjected to signal processing by a PC 415. Thus, according to this method, data is actually written to each picture element for inspection, whereby it is possible not only to inspect the condition of the driving circuit and the bus lines but also to detect any defects in the picture element transistors on the active matrix substrate.
With this method, however, a video line that is used for writing data to a picture element needs to be used also for reading out the written data therefrom. Therefore, the inspection method requires a circuit that allows for reversible signal flow. Specifically, in a case where the driving force of the source line driving circuit is smaller than the load of the source line, e.g., with a large-sized panel or a high-definition panel, it is necessary to provide an amplifier 502 at the final output stage for outputting data to a source line, as illustrated in FIG. 18. Since an amplifier is not a circuit that allows for reversible signal flow, the data written to a picture element from a video line 501 cannot be read out therethrough.
Moreover, a digital driver as illustrated in FIG. 19 requires a DA converter 601 for converting a video digital signal into an analog voltage for liquid crystal display. However, the DA converter 601 is also not a circuit that allows for reversible signal flow, and cannot read out data that has been written to a picture element.